Anticipatory control of headbox slice opening in a paper machine

ABSTRACT

A HEADBOX OF A MACHINE PRODUCING A FIBROUS SHEET HAS A SLICE OPENING CONTROLLED IN RESPONSE TO SIGNALS INDICATIVE OF TOTAL HEAD OF FLUID IN THE HEADBOX, THE RATE OF FLOW OF A FIBROUS SLURRY INTO THE HEADBOX AND THE SPEED OF A FOURDRINIER WIRE RECEIVING A JET OF SLURRY EMERGING FROM THE HEADBOX SLICE. THE SLICE OPENING IS CONTROLLED TO AFFECT FORMATION OF THE FIBROUS SHEET ON THE WIRE. CONSISTENCY OF THE SLURRY BEING FED INTO THE HEADBOX IS VARIED TO CONTROL THE POSITION OF A DRY LINE ON THE WIRE OF FORMATION OF FIBERS ON THE WIRE. THE LEVEL OF LIQUID IN THE HEADBOX IS MAINTAINED AT A PREDETERMINED LEVEL BY A CONTROLLER. THE SLICE OPENING IS CONTROLLED IN ANTICIPATORY MANNER TO COMPENSATE FOR THE SLOW RESPONSE OF THE TOTAL HEAD IN THE HEADBOX TO CHANGES OCCURING IN RESPONSE TO THE SLURRY MASS FLOW RATE INTO AND OUT OF THE HEADBOX. THE CONTROLLER FOR THE SLICE OPENING IS PROVIDED WITH DEADBAND.

Nov. 21, 1972 J. s. RICE 3,703,436

ANICIPATORY CONTROL OF HEADBQX SLICE OPENING IN A PAPER MACHINE Filed Feb. 5. 1970 3 Sheets-Sheet z F/6.3a QsTRM 1- T I J T. H635 H t 1 /6152 Us 3 P H6. 3d hsL T (\DEAL) T x it 4 Q I F/6.3e H L (\DEAL) T T I 't INVENTOZI JAMES 5. F/CE wi k Arm/ave Y5 United States Patent 3,703,436 ANTICIPATORY CONTROL OF HEADBOX SLICE OPENING IN A PAPER MACHINE James S. Rice, Columbus, Ohio, assignor to Industrial Nucleonics Corporation Filed Feb. 3, 1970, Ser. No. 8,292 Int. Cl. D21f N06 US. Cl. l62198 12 Claims ABSTRACT OF THE DISCLOSURE A headbox of a machine producing a fibrous sheet has a slice opening controlled in response to signals indicative of total head of fluid in the headbox, the rate of flow of a fibrous slurry into the headbox and the speed of a Fourdrinier wire receiving a jet of slurry emerging from the headbox slice. The slice opening is controlled to affect formation of the fibrous sheet on the wire. Consistency of the slurry being fed into the headbox is varied to control the position of a dry line on the wire or formation of fibers on the wire. The level of liquid in the headbox is maintained at a predetermined level by a controller. The slice opening is controlled in an anticipatory manner to compensate for the slow response of the total head in the headbox to changes occurring in response to the slurry mass flow rate into and out of the headbox. The controller for the slice opening is provided with deadband.

The present invention relates generally to systems for and methods of controlling machines for forming fibrous sheets and, more particularly, to such a control system and method wherein a headbox of the machine is controlled in response to signals indicative of the flow rate of a fibrous slurry to the headbox and the total head of fluid within the headbox.

In the formation of a fibrous sheet, such as paper, a headbox includes a slice for depositing a jet of liquid fiber slurry onto a traveling Fourdrinier wire, which functions as a means for removing water from the slurry. The relative velocity between the jet spouting from the slice opening and the wire determines the formation of the resulting sheet. The relative velocity between the jet and wire is referred to as either the jet-to-wire ratio or the rushdrag; the term jet-to-wire ratio is the quotient of the jet spouting velocity divided by wire forward speed, while rush-drag is indicative of the difference between the jet and wire velocities. For a rush condition or a jet-to-wire ratio greater than one, fiber bunching on the wire occurs in such a manner as to cause the formation of ripples in the sheet. For a drag condition, wherein the jet-to-wire ratio is less than one, the fibers spread longitudinally on the wire and a sheet having no ripples therein is formed. Typically, the jet-to-wire ratio is maintained in the region of between 0.98 and 1.02, depending upon the particular type of sheet desired to be formed. Hence, to control accurately the type of paper being fabricated by a particular fibrous sheet producing facility, it is important to control the relative velocities of the wire and jet emerging from the headbox slice opening.

Another important consideration in the operation of a headbox utilized in forming a fibrous sheet is to maintain the level of liquid in the headbox at a predetermined height. For certain types of paper, the liquid level should be above the top of all holey rolls within the headbox, while for other types of paper the liquid level in the headbox should be set at other points. Modern headboxes are closed and include pneumatic systems to exert positive or negative air pressures on the liquid within a closed headbox. Sensors of the total head and air pressure within the headbox derive signals which control the positive and ice negative pressures applied to the headbox for liquid level control. As the total head within a headbox is varied to provide liquid level control, the velocity of the jet emergrug from the headbox slice opening changes. Thereby, assummg a constant wire speed, the jet-to-wire ratio or rushdrag is changed to vary the formation of the sheet.

Another factor aifecting the operation of a paper making facility is the consistency of fibers in the fiber-water slurry fed into the headbox, which controls the consistency of the slurry in the headbox and emerging from the headbox. Consistency is one of several factors controlling the point on the Fourdrinier wire where a so-called dry line forms. The dry line is that point on the wire where no water can be removed from the slurry by gravity. It can be determined visually by a paper maker since there is no water on the surface of the mat formed from the slurry and must occur prior to the end of the Fourdrinier wire remote from the headbox slice opening. In many instances, it is also desirable to have the dry line located at a particular point on the wire so that a dandy roller on the wire can be effectively utilized. If the dry line is formed upstream of the dandy roller, fibers will become caked thereon and an unacceptable sheet will be formed. Consistency of the slurry jet emerging from the headbox is also important to some paper makers because it affects formation properties of the fibers on the wire. If consistency is used to control formation, it cannot also be employed to controllable position the dry line and vice versa.

To control the consistency of slurry fed to a headbox and hence the position of the dry line or fiber formation on the wire, the rate of flow of water relative to fiber rate of flow into the headbox is controlled. Typically, this is accomplished by maintaining the fiber fiow rate into the headbox constant, while varying the amount of water fed into the headbox opening. Preferably, consistency of the slurry in the headbox is not varied by controlling the total amount of fiber fed into the headbox because total fiber into the headbox is equal to the fiber in the resultant sheet, a quantity that is generally fixed for a particular type or grade of paper.

From the foregoing, it is seen that it is important to control the rush-drag, headbox level and headbox consistency of a machine forming a fibrous sheet. In the prior art, to my knowledge there has been no attempt to control in a concomitant manner these three variables associated with the formation of a fibrous sheet. Certain workers in the field have employed or propose to employ systems wherein rush-drag and headbox level are controlled, while others have proposed systems for varying headbox consistency.

I have found that complete control of the formation of a fibrous sheet can most advantageously be effected through control of the three parameters, rush-drag, headbox level and headbox consistency. These parameters are interrelated with each other in a relatively complex manner and are controlled, in accordance with the present invention, by providing headbox actuators for the ratio of white water to fiber fed into the headbox, the level of liquid in the headbox and the extent of slice opening. The consistency of the fiber-water slurry fed into the headbox is preferably controlled by varying the setting of a stream valve feeding the slurry into the headbox, but it is to be understood that it can also be controlled by varying the speed or capacity of a fan pump responsive to input slurry and white water from a wire pit.

The three headbox actuators energized to determine the three headbox variables of rush-drag, headbox level and headbox consistency control the stream valve, slice opening and pressure system for level control. Because three variables are responsive to the energization of three actuators, complete control of the headbox can be attained in a coordinated manner.

For certain fibrous sheet formation machines, it is desirable to provide one or the other of the actuators with a significant amount of dead band. For example, some paper makers object to varying the headbox slice opening except when absolutely necessary, while others may choose to insert a considerable degree of deadband in the dry line and still others may permit the liquid level within the headbox to vary significantly prior to actuating a control. Because three variables are controlled in response to energization of three separate independent actuators in the present invention, wide tolerance in the selection of the actuator to have a significant amount of headband can be obtained. In the specifically disclosed embodiment, the slice opening is selected to have a significant headband, whereby changes in signals controlling the slice opening actuators must exceed a predetermined limit prior to be effective to energize the slice opening actuator.

A problem in controlling the actuators of a headbox to enable the paper making machine to produce a sheet having homogeneous properties resides in the different speeds of response of the various elements. For example, stream valves feeding slurries into the headbox can be changed at a relatively rapid rate and have a short time control. In response to changes of the flow rate of slurry into and out of the headbox, the liquid level and total head within the headbox have longer time constants than the valve time constants. The response time of an actuator for controlling the extent of a'slice opening is relatively slow and different from the response times of headbox liquid level and total head changes, as well as the response time of the stream valve actuator. Because of these different response times, the various actuators generally cannot be controlled in a straightforward manner without substantially changing the properties of the sheet on a transient basis if a change in one of the parameters is purposely introduced or occurs in response to an internal change in the machine or the slurry fed thereto. In accordance with another aspect of the present invention, this problem is obviated by providing a quasimathematical model of a headbox. In accordance with a specifically disclosed embodiment, the slice opening actuator is energized in response to the model to minimize variations in fiber sheet formation occurring in response to changes in rush-drag. The rush-drag changes can be responsive to variations in the total amount of slurry fed into the headbox; wire velocity; slice opening; total headbox head; or an operator induced variation.

It is an object of the present invention to provide a new and improved system for and method of controlling the formation of a fibrous sheet.

Another object of the present invention is to provide a new and improved system for controlling a headbox of a fibrous sheet formation machine, wherein deadband in one of the actuators can be provided.

Another object of the present invention is to provide a new and improved system for controlling a headbox wherein headbox consistenc'y, rush-drag and liquid level in the headbox are controlled in a coordinated manner.

Another object of the present invention is to provide a new and improved system for controlling a headbox wherein dry line, rush-drag and liquid level in the headbox are controlled in a coordinated manner.

Still another object of the present invention is to provide a headbox controller wherein compensation for the different speeds of response of the various actuators and parameters is provided to attain an anticipatory type of control.

Another object of the present invention is to provide a new and improved system for controlling a headbox of a fibrous sheet formation machine, wherein the headbox slice opening is controlled in response to signals indicative of total fluid head in the headbox and the consistency of fiber flowing into the headbox.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a functional block diagram illustrating one embodiment of the present invention;

FIG. 2 is a flow diagram of the operation of the computer illustrated in FIG. 1; and

FIGS. 3a-4d are waveforms useful in describing the system of FIG. 1 and the flow diagram of FIG. 2.

Reference is now made to FIG. 1 of the drawings wherein a closed, pressurized headbox 11 is illustrated. Headbox 11 is supplied with a fiber-water slurry through conduit 12 and feeds a jet of the slurry from a slice opening 13 thereof onto Fourdrinier wire 14. Fourdrinier wire 14 removes liquid from the slurry by gravity and suction means (not shown) and may be provided with a dandy roller 15. Sufficient water is removed from the slurry on wire 14 to enable a dry line to be formed on the wire.

Water drained from the slurry on wire 14 is caught in pit 16 and is returned as white Water to pump 17 via line 18. The white water in line 18 is combined with thick stock coupled through valve '19 and conduit 20, from constant pressure source 21. The slurry emerging from pump 17 is fed through stream valve 22 and conduits 23 and 12 to the inlet of headbox 11. Stream valve 22 functions to control the consistency of the slurry fed by line 12 into headbox 11. For any particular fiber content of the sheets to be ultimately formed, valve 19 is set and is not varied. Thereby, the fiber flow rate through valve 19 and line 20 into pump 17 is relatively constant while the amount of white water fed from pit 16 to pump 17 via line 18 is variable, depending upon the setting of valve 22. Since the fiber flow rate through valve 22 is constant and the white water flow rate is variable, the consistency of the slurry fed into headbox 11 is controlled in response to variable quantities of white water fed back to pump 17. Thereby, as the consistency of fiber in line 12 varies, the level of liquid in headbox 11 also has a tendency to change.

To control the level of liquid pond formed in headbox 11, and preferably maintain it at a constant level for any particular grade and/ or type of sheet, a feedback loop is provided to control the pressure of an air pad in the top of the headbox for applying a force to the liquid pond. The feedback loop includes pressure transducer 24, located at the bottom of headbox 11, for deriving an output signal directly proportional to the total head of the liquid pond and fluid pad within the headbox. The air pad pressure is monitored by pressure transducer 25, located towards the top of headbox 11 above the liquid level. The pressure indicating output signals of transducers 24 and 25 are combined in a subtraction network 26, the output of which is a signal indicative of the level of the liquid pond within headbox 11. The output of difference network 26 is combined in subtraction network 27 with a signal indicative of desired liquid level, determined by the grade and type of paper being formed by an operator or from a computer memory.

The error signal derived from subtraction circuit 27 is fed to valve actuator 28 which controls three-way valves 29 and 30. One port of each of valves 29 and 30 is connected to the inlet and outlet, respectively, of centrifugal pump 31, while other ports of the valves are coupled to the air pad within headbox 11. The remaining ports of valves 29 and 30 are coupled to the atmosphere. In a manner well known to those skilled in the art, valves 29 and 30, in combination with the actuator 28 therefor and pump 31, function to exert positive and negative pressures on the liquid pond in headbox 11 to decrease and increase the pond height. In response to the height of the pond increasing above the set point value therefor, the output of difference network 27 is positive to open valve 29 and close valve 30, whereby the air pad within headbox 11 has an increased pressure to force the liquid downwardly and reduce the pond height. In an opposite manner, in response to the level of the pond being less than the target value therefor, valve 29 is closed and valve 30 is opened to provide suction or decreased pressure on the air pad and thereby raise the liquid level of the pond.

To control the consistency of the fiber-water slurry fed by line 12 into headbox 11, and hence the position of the dry line on wire 14, the position of valve 22 is controlled in response to a signal stored in counter 33. Counter 33 derives a multibit, digital output signal that is transformed by digital-to-analog converter 34 into an analog control signal for valve 22. Digital-to-analog converter 34 feeds one input of subtraction network 35, the other input of which is responsive to a smoothed signal derived from magnetic flow meter 36. The output of flow meter 36 is smoothed because the flow meter output is inherently very noisy. A smoothing circuit, such as a low pass filter, removes a sufficient amount of the noise to accurately control valve 22. In response to the difference between the flow indicating output signal of meter 36 and a target value for the flow through valve 22, as derived by digital-to-analog converter 34, difference network drives actuator 137 for valve 22. Thereby, the flow rate through valve 22 is maintained at a target value determined by the output of counter 33.

The output of counter 33 is initially set in response to an indication of the grade and type of paper being manufactured. To this end, the stages of the counter are set in response to binary signals indicative of the consistency which is expected to provide a dry line position upstream of dandy roll 15. Under certain operating conditions, the paper maker desires to control the position of the dry line on wire 14. For example, the type of fiber fed by source 21 is susceptible to variations which can result in movement of the dry line but the paper maker generally desires to maintain the dry line position substantially constant despite these fiber variations. To enable the dry line position to be controlled, the state of counter 33 can be controlled by an operator, using a manual control, in positive and negative going directions.

To this end, counter 33 includes count up and count down input terminals, selectively responsive to the output of constant frequency pulse oscillator 39. Pulse oscillator 39 is selectively connected to terminal 37 and 38 via manually actuated, operator controlled switches 41 and 42, respectively. An operator activates switches 41 and 42 for variable times, depending upon and proportional to the amount by which he desires to translate the position of the dry line on wire 14. If it is desired to translate the dry line towards slice opening 13, switch 41 is closed, while switch 42 is activated to the closed state if the operator desires to translate the dry line away from the slice opening 13. For each second that switches 41 or 42 are activated to the closed state, a pulse is supplied by source 39 to counter 33 to drive the counter to lower or higher states.

To maintain bone dry basis weight, i.e., fiber content per unit area of formed sheet, relatively constant, a feedback loop is provided between a target value therefor, as derived on lead 43, and the output of flow meter 44 that monitors flow rate in line 20 and thereby provides an indication of fiber fed to headbox 11. The signal on lead 43 is derived in response to either a stored signal from a computer memory for a particular grade and type of paper or from an operator input signifying a particular bone dry basis weight. In the alternative, the bone dry basis weight signal can also be derived from automatic feedback control means responsive to gauging means downstream of Fourdrinier wire 14, in the vicinity of a .takeup roll (not shown). The relatively constant signal on lead 43 is combined with a smoothed signal indicative of flow rate through valve 19, as derived from fiow meter 44. The actual and target values for bone dry basis weight or fiber flow through valve 19 are compared in subtraction network 45, the output of which feeds actuator 46 for valve 19.

To control the forward velocity of wire 14, the wire is attached to hub 51, which is driven by motor 52- that is responsive to a speed control output voltage of motor speed control network 53. The rotational velocity of output shaft 54 of motor 52 is controlled in response to the D0. voltage applied to motor speed control network 53 by tap '55 of potentiometer 56, having terminals connected to a suitable voltage source. The speed of wire 14 is measured by monitoring the rotational velocity of shaft 54 with tachometer generator 57, having an armature that is mounted on shaft 54. To control the height of slice opening '13 uniformly across the entire width of headbox 11, a slice drive 61 and its actuator 62 are provided. Slice drive actuator 62 is responsive to the output of difference network 63, having one input responsive to position transducer 64 that monitors the height of slice 13 and a second input that is a set point signal for the slice opening height. The difference output signal of network 63 drive actuator 62 to control the height of slice opening 13. The set point signal derived on lead 65 is derived from digital-to-analog converter 66, which in turn is driven by an output signal of computer 67.

Computer 67 is preferably a general purpose, digital computer including a memory, arithmetic unit, inputoutput units and transfer buses, and may be any one of a number of suitable scientific type computers available on the market. In one actually built embodiment of the present invention, computer 67 is an IBM 1800 machine. For purposes of simplicity, computer 67 is shown as deriving a set point signal for actuator 62 that drives slice drive 61 to control the height of slice opening .13. In actuality, the computer also includes means to establish the setting of valves 19, 22, 29 and 30' to control the bone dry basis weight, consistency and level of the liquid in headbox 11. The computer also preferably includes digital-to-analog and analog-to-digital converter programs to obviate the need for external converters. Computer 67 also may include means for controlling the voltage applied to motor speed control network 53 to vary automatically the velocity of Fourdrinier wire 14.

Computer 67 can be employed to control the consistency of slurry in headbox 11, in an alternative embodiment wherein consistency is varied to control fiber formation on wire 14, rather than the dry line position. In the alternate embodiment, an operator supplies an input to computer 67 indicative of desired value of headbox consistency and the computer stores the value as a signal in a memory thereof. The computer memory also stores a signal indicative of flow rate through stream valve 22 and feed this signal via the connection shown by dotted line 68 to digital-to-analog converter 34, thereby to control headbox consistency. When the system is operating in the alternate mode, neither converter 34 nor computer 67 is responsive to the dry line control output of counter '33 because of the mutually exclusive nature of control for dry line and fiber formation as a function of consistency. In some systems, it may be desirable to include partial control of both dry line position and fiber formation as a function of headbox consistency, with exclusive control of neither a feature included within the purview of this invention. In response to the operator desiring to make a change in headbox consistency to vary fiber formation, a signal indicative of the new value of headbox consistency (C is supplied to computer 67 and the computer calculates a new stream valve set point (Q in accordance with:

H QBTRMNEW= QSTRMOLD (6&2 NEW where QSTRMOLD= the previous set point for stream valve 22 calculated by computer 67; and

C =the value of consistency previously fed into the computer by the operator.

In response to the new stream valve set point, actuator 137 drives stream valve 22 to vary the stream mass flow rate and enable the desired headbox consistency to be achieved.

The computer also responds to input signals indicative of sheet bone dry basis weight (BDBW), determined by suitable means, a stored set point for stream flow (Q derived from a stream valve set point, wire speed, V as derived from tachometer generator 57, and suitable constant factors, K, to calculate actual headbox consistency, U as:

Qsrmu The calculated value of U is fed to a display 69 and can be compared by the operator with a desired value of headbox consistency, C

In the illustrated embodiment, one of the functions of computer 67 is to derive a control signal for the set point of slice opening 13. To this end, computer 67 responds once every second (the computer cycle time) to each of: the wire velocity indicating output of tachometer generator 57, the total head output signal of pressure gauge 24 and the set point value for stream valve 22. In addition, the computer responds during each one second cycle period to a signal retrieved from the memory thereof, which signal is indicative of a set point for rushdrag. The value of the rush-drag set point is dependent upon the grade of the particular type of fibrous sheet being formed and/or an operator induced value therefor. In response to the rush-drag set point, total head and stream valve signals, as well as other signals retrieved from memory, computer 67 is programmed to calculate slice height set point.

A direct, straightforward manner for calculating slice height set point is not performed because of the dynamic properties of headbox total head changes, slice movement and stream valve 22, The headbox total head has a long time constant because variations in the total head within headbox 11 occur a significant time period after valve 22 is changed. The slow response of the total head occurs because of the relatively large liquid capacity of the headbox and the relatively long time required to change the air pad pressure. The slice 13 is translated from one position to another for a time period generally different from the time required to change headbox 11 total head and/or stream valve 22 position. To compensate for these diverse actuation times and time constants and enable a sheet having relatively constant, controlled properties to be formed, the program of computer 67 includes an arithmetic model to anticipate the response of the headbox to changes in the flow of slurry through stream valve 22 and slice 13. Compensation for the headbox dynamic response is reflected in the set point signal derived by computer 67 for the opening of slice 13.

The set point signal for the height of slice opening 13, neglecting the anticipatory type control, is computed in accordance with:

where Ah is the change in the height of slice opening 13 from a previously computed value thereof,

h is the previously computed set point value for the height of slice opening 13,

R is the set point value for rush-drag,

v is the velocity of wire 14, as derived from tachometer 57, and

AR is the error in the rush-drag.

It can be generally stated that the rush-drag error, AR can be computed, neglecting the anticipatory control, in accordance with:

where g is the gravitational constant, and H=the total head in headbox 11, as derived from pressure transducer 24.

Physically, Equation 2 states that the rush-drag error, that is, the ditference between the set point value for rush-drag and the actual value of rush-drag, is equal to the rush-drag set point R minus the actual rush-drag. The actual rush-drag is equal to the difference in the velocity of the jet emerging from slice opening 13 and the velocity of wire 14. The velocity of wire 14, v is derived by tachometer generator 57 directly. The velocity of the jet emerging from slice opening 13 is approximated accurately at the vena contracta as /2gH. It is thus evident that Equation 2 merely restates, in mathematical terms, the rush-drag error as being equal to a set point for rush-drag minus an actual rush-drag value.

Physically, Equation 1 states that the ratio of a change in the set point for slice opening 13 to the actual value of the slice opening height set point in the velocity (v of the jet emerging from slice opening 13. This results from the definition of It follows that:

Ah sE'r E SLs VBPOUT Equation 2a is not completely accurate because it does not completely reflect changes in the value of AR that are added to VSPOUT in the denominator. Since AR is much less than V the approximation is considered valid. If the velocity of wire 14 and rush drag set point both remain constant, the only factor contributing to AR is a variation in the total head of the fluids in headbox 11. Hence, another manner for viewing Equation 1 from a physical aspect is that the ratio of changes in the height of slice opening 13 divided by the value of the slice opening height sL sr.)

is directly proportional to the ratio of changes of the total head to the total head in headbox 11 (AH/H) From a physical reasoning, this is the expected result which can be proven mathematically; there is believed no need, however, for a mathematical proof of this conclusion.

The signal applied by computer 67 to digital-to-analog converter 66 to control the set point for the opening of slice 13 is provided with a certain amount of deadband. Deadband is introduced so that the set point signal on lead 65 does not change unless the calculated value therefor in computer 67 exceeds a predetermined change. If the calculated value for the height of slice opening 13 does not exceed the predetermined change, the set point signal coupled by computer 67 to digital-to-analog converter 66 remains constant and slice opening 13' is not varied in position.

It is considered advisable in certain headboxes, to minimize the number of times slice opening 13 is activated and thereby provide a certain deadband for activation of the slice, to prevent vibration and wear as the slice is moved. By providing significant deadband in the set point deriving signal these problems are substantially obviated.

The actual value for rush-drag set point, and hence slice opening set point, is computed by compensating for the slow response time of the headbox to changes in the setting of stream valve 22 and slice opening. In particular, if a change in the setting of stream valve 22 occurs during a one-second cycle time of the program of computer 67, that change is divided by the present value of the stream valve setting to form a signal indicative of stream flow percentage change as given by the ratio:

Q STRM where:

QSTRM is the value of the stream valve setting or set point during the computer program cycle under consideration, and

QSTRMOLD is the setting or set point value for the stream valve 22 during the computer cycle completed immediately prior to the cycle under consideration. The ratio of Expression 3 is multiplied by a value indicative of jet velocity during the computer cycle period being considered to derive an indication of rush-drag set point change. The jet velocity is calculated in response to a set point value for rush-drag and the velocity of wire 14 during the cycle period being considered in accordance with:

whereby the change in rush-drag set point between successive computer cycles is a function including, inter alia, the term:

( R SET w) Q S'IRM Q STRMOLD) QSTRM (5) The anticipatory control for s-lice opening height is derived in response to rush-drag set point changes indicated by Expression 5 and a filtered value for total head. The filtered head value is calculated by computer 67 in response to the output of gauge 24, which is fed to the computer once every second. Once every fifteen cycle times a new filtered head value signal, H, is established for indicating the value of headbox jet spouting velocity in calculating rush-drag error. Because of the slow response of the headbox liquid level and hence total head therein, to changes in stream flow and slice opening, the change in rush-drag set point calculated in response to the value of H sampled once every fifteen cycle times may not always provide an accurate method for calculating slice height set point. To obviate this possibility, the computer program includes a portion for modeling the headbox head variation-s in response to slice height set variations. In the particular model, the calculated changes in slice height set point are accumulated over a predetermined number (less than a sampling interval) of computer cycles. If changes in slice height set point occur during the latter portion of a sampling interval, they are effectively subtracted from the value of H in the calculation of slice height set point during each of the cycle times over the next sampling interval. Since changes in stream valve are reflected in rush-drag set point, which is also affected by H and controls the slice height opening set, compensation for the different dynamic characteristics 10 of the headbox elements is derived by the technique, as well become more clearly evident infra.

Prior to considering the specific program for controlling the set point of slice opening 13, h some general comments regarding the computer program and memory are deemed in order. The computer memory includes a plurality of slots for storing data words fed thereto in binary form from an analog-to-digital converter in the computer, which converter is time multiplexed with the several transducers illustrated in FIG. 1. In certain cases, a multiplicity of memory locations is provided for storing data Words derived and repeatedly used during a number of computer cycle periods, while for certain of the data words a single memory location is provided because they are utilized only during one cycle period. The computer memory also includes an index register or counter which is incremented once during every one second program cycle time and is reset to one after fifteen one-second cycle times have been completed. The computer memory also includes a program for controlling data word sampling and transferring data Words between memory locations and an arithmetic unit, as Well as subroutines for directing the computer to perform certain arithmetic operations on the data Words.

In considering the computer program flow chart, FIG. 2, standard Fortran notation will be utilized. In Fortran notation, an equals sign signifies the transfer of information into a designated memory slot, a rectangle indicates a transfer or subroutine operation, and a diamond indicates a decision operation. The operations and decisions are performed in a manner well known to those skilled in the computer art and the specific manner by which the machine performs them need not be described for an understanding of the invention.

Prior to any of the calculations of slice opening set point 13 being performed, the set point for the flow through stream valve 22, Q is fed into the computer memory once each one second cycle period. Because of the fast response time of valve 22, the set point is considered equivalent to actual flow into headbox 11 and a more accurate representation of flow than the signal of meter 36 because of the meter noise characteristics. The memory also stores the value of QSTRM fed to the computer during the just completed one-second cycle period. In addition, the computer samples and stores the output of tachometer 57, a signal indicative of the speed (v of Fourdrinier Wire 14, during each computer cycle period. The value of v is replaced during each one second computer cycle period as there is no need to store previous values thereof. The value of rush-drag set point, as entered into the computer from an operator station or by the computer memory from a grade change program, is sampled and stored once during each one second cycle time, as is the rush-drag set point from the just completed one second cycle period. The total head signal, H, derived from pressure gauge 24, is read into the computer memory once every second. The computer is programmed to calculate a filtered total head value, H". The value of H" is sampled once every fifteen cycle times to derive a sampled, filtered total head signal, H, which enters into the calculation of AR once every fifteen cycle times and remains constant over that period, frequently referred to herein as a sampling interval. Through the filtering and sampling processes the slow response time of the headbox level characteristics are effectively simulated or modeled.

After the data words indicative of the responses of the various elements have been fed into and stored in the computer memory, operations associated with calculating slice opening commence. The first operation performed by computer 67 in calculating the set point for the height of slice opening 13 during each one second cycle period involves calculating filtered total head in accordance with the operation indicated in FIG. 2 by rectangle 71 as:

'he constants a and b are filtering constants defined a=e and b=1e where e is the base of natural logarithms,

t is the length of each computer program cycle time, one

second in the present instance, and

-r is a filtering time constant selected to attenuate noise in total head measurement derived from pressure gauge In operation 71, the value of filtered total headbox head, H", for a particular one-second computer cycle being considered is calculated in response to the value of total head computed during the just completed computer program cycle time, H", multiplied by the filtering constant a The a H" product is added to the product of total head, H, as derived during the cycle being considered and the filtering constant, b The a H" and b H terms are added together by the computer arithmetic unit in response to values retrived from memory and are stored in the computer memory in the location where H" was stored during the previous computer cycle time.

After the operation indicated by rectangle 71 has been completed, the computer program steps to the decision operation indicated by diamond 72. In decision operation 72, the index counter of the computer memory is examined to determine if a fifteen cycle or second sampling interval has been completed. If the index counter is set at fifteen, a yes decision is made and the program proceeds to the operation indicated by rectangle 73, whereby the index counter is reset to a state of one. Following the operation 73, the computer program advances to the operation indicated by rectangle 74, whereby the filtered total head signal calculated during operation 71 is stored in the memory slot for H. The value of H is indicative of a value of total head used in calculating rush-drag error during the next fifteen cycle times, i.e., one sampling interval. Upon completion of operation 74, the computer program is activated to execute operation 75, setting the value of rush-drag set point (R derived from an operators station or a grade value thereof as exists in memory, during the cycle being considered, into a memory location wherein an updated set point (R' for rush-drag is stored. In response to operations 74 and 75 and the stored value for wire speed (V during the one second cycle being considered, rush-drag error (AR is computed in accordance with computer subroutine arithmetic operation 76,

The diflFerence between the equation solved during operation 76 and Equation 2 is that in the former an updated value of rush-drag and a sampled, filtered total head are utilized.

Between the fifteen second sampling intervals of the computer program, operation 72 causes a branch in the computer program to be executed, as indicated by the no symbol to the right side of the diamond. The branch is entered during every one second program cycle time, except when a yes decision is made indicating that the index counter is set to fifteen. In response to a no decision by element 72, the index counter is advanced by a count of one, as indicated by operation 77.

After the index counter has been incremented, the rushdrag set point is updated or modeled mathematically during each cycle over the fifteen cycle sampling interval, in accordance with the subroutine arithmetic operations 78. Operations 78 involve calculating an updated value for rush-drag set point (R in response to the value of rush-drag set point (R' calculated during the just previously completed one-second computer cycle time. The

previously computed rush-drag set point is added to the rush-drag set point (R fed into the computer during the presently considered cycle. The sum (R -I-R has subtracted from it the value (R of rush-drag set point fed into computer during the just previously completed computer cycle time. Normally, the quantity R -R is zero and has a finite value only in response generally to an operator induced change in rushdrag set point or during grade change. The quantity (R' -l-R R has subtracted from it the rushdrag error, AR computed during previously completed computer cycle time.

The next portion of arithmetic operation 78 involves calculating the rush-drag set point change which should be made because of an operator or grade change variation in the consistency of the slurry fed to headbox 11, which ultimately controls the dry line position on wire 14. It is to be recalled that changes in the consistency of the fiberwater slurry flowing into headbox 11 are controlled in response to movement of stream valve 22. In response to the change in the set point (Q for stream value 22 between the cycle being considered and the set point (Q for the stream valve during the cycle period which occurred just prior to the cycle now being considered, the percentage change in the stream valve setting is calculated in accordance with:

Q STRM Q STE-M The percentage change indicated by Expression 6 is multiplied by a value indicative of the desired or set point velocity (v of the jet emerging from slice opening 13. The jet velocity set point is calculated as (R' -l-v in response to values of R' and v stored in the computer memory while operation 78 is being performed. The percentage change in stream flow into headbox 11 is multiplied by the set point for the jet velocity emerging from slice opening 13 to provide, in combination with the values of RISET, RSET, RSETOLD and ARBOLD, an updated or anticipatory or compensated value for rushdrag set point in accordance with Q STRMSET SET SET+ SET SETQLD G SET+VW) Q STRM SET Q STRMOLD Q STRMSET The mathematical expression calculated during operation 78 serves as a model of the headbox to indicate the manner by which the headbox responds to changes in the rate of flow of the fiber-water slurry fed thereto or to changes in rush-drag set point induced by an operator. Operation 78 indicates the manner by which the rush-drag set point changes in response to changes in stream flow and changes in rush-drag set point.

After operation 78 has been completed, the set point for stream valve 22 derived during the presently considered cycle is relocated in the computer memory at the location previously occupied by the stream valve set point for the previously considered operating cycle, as indicated by operation 79.

Upon completion of operation 79, the computer program branch associated with decision operation 72 is terminated and the program enters operation 76 wherein rush-drag error is calculated in response to the value of R' as calculated in the operation 78 or as set during operation 75. Operation 76 for calculating rush-drag error (AR is performed in response to the value of R computed in operation 78 for each cycle when the no decision of operation 72 is made; the value of AR in operation 76 is performed in response to the value of R' established in operation 74 during those cycles when operation 72 provides a yes decision. While the R' signal in operation 76 is susceptible to change during each cycle time, the H signal therein can only change once every fifteen cycle times, while the yes branch of v operation 72 is executed. Hence, the rush-drag error (AR is )updated in response to continuously updated values of rush-drag set point (R but is responsive only to sample values of total head and does not follow transient or filtered values of total head. The rush-drag error is calculated in response to sampled values of total head, rather than current values thereof, because of the delay of the pond in headbox 11 changing height in response to changes of stream flow into the headbox.

The next operation in the computer program is to determine the amount by which the set point for the slice opening should be moved, as indicated by operation 77. The amount by which the slice opening set point should be changed, Ah is calculated in response to the rush-drag error (AR multiplied by the ratio of the slice screw height set point (h calculated during the previous computer cycle period to an updated, compensated value for slice opening set point. The compensated value for slice opening set point is calculated in response to the value of wire speed, v fed into the computer during the cycle being considered, and the updated rushdrag set point, calculated in accordance with operation 78. The terms R and v are added together to provide the updated value for slice set point opening and are combined with hSLSET and AR in accordance with h SLSET as indicated by the subroutine of operation 77.

After the change in the height of the slice set point has been calculated during operation 77, the change is tested to determine if it exceeds a deadband limit, indicated by decision operation 178. In operation 178, the height for the set point of slice 13 (h calculated during the previously considered computer cycle time is multiplied with a predetermined constant (A) inserted into the computer memory. The factor (A) is an a priori determined deadband factor representing a fiaction of the slice opening less than a percentage which the slice should not be translated. The product of (A) and the previously computed slice opening set point [(A) h is compared with the change in slice set point (Ah calculated during operation 77. If the calculated change in slice screw set point is greater than or equal to the product of the deadband factor and the previously computed slice opening set point, the Ah term computed in operation 77 is properly utilized to enable the slice set point to be changed. If, however, the calculated change in the slice opening set point is less than the product of the deadband factor and the previously computed slice set point, the computer program enters a branch and operations 179' and 80 are performed to nullify the calculations made during operations 76 and 77. In operation 179 the change in the set point of the slice opening is set to zero, while in operation 80 the rush-drag error is set to zero.

Upon completion of operation 80 or a no decision during operation 178, which no decision does not require a branching operation, the index counter is examined, as indicated by diamond 82. In response to the index counter being set to one, as occurs once every fifteen computer cycle times when a yes decision is made during operation 72, the computer executes branch operation 83. Operation 83 involves calculating a change in set point for slice opening in response to change in slice opening set point computed during the just previously completed cycle minus the accumulated slice set point changes during the last six cycles of a fifteen cycle sampling interval. The manner by which the slice set point changes are accumulated is set forth infra, with regard to operation 84.

The accumulated slice set point changes during the last seven cycles of a sampling interval provide a compensation for the total head change resulting from Slice change which is not reflected in the value of H when the index counter is set to one at the beginning of each sampling interval. The compensated total head due to slice movement enters into the calculation of Ah once every fifteen cycles for those compensated changes made during the last half approximately of the sampling interval because the calculated changes in slice set point have not yet generally aifected filtered total head H" when H is set to H at the beginning of a sampling interval. In contrast, the changes in slice opening set point calculated during approximately the first half of a sampling interval have caused the total head to change sufiiciently by the time a new sampling interval begins to be reflected in the calculated, filtered head at that time to not require the compensation provided by operation 83.

The calculations for compensated total head due to slice movement are performed once during each computer cycle operating time, regardless of the state of the index counter. The computer program enters operation 84 either directly after decision operation 82 or calculation operation 83.

In operation 84, there is calculated an anticipatory or compensated value for the movement of the pond in the headbox in response to changes in slice opening 13. The calculation is performed over an interval of six one second computer cycle times. Operation 84 involves calculating the value of compensated total head Ah(8-i) due to slice movement over the last seven computer operating cycles in response to the value of Ah determined during operation 77 or 79 and the accumulated changes of slice set point changes, Ah (7'i) over the six previous cycles as:

wherein the value of 1' runs from one to six.

Upon completion of operation 84, operation is performed to set the value [Ah(1)] of compensated head in operation 84 for the next cycle. The value for Ah(l) in operation 85 is set in response to the slice set point change derived during one of operations 77, 79 or 83, whichever one caused the most recent change in the value of Ah The value of Ah(1) derived during operation 85 replaces the value of Ah(l) previously utilized during operation 84. The value of Ah(1) previously utilized during operation 84 is stepped to Ah(2). Stepping between the various values of Ah in operation 84 occurs in this manner until the value of Ah(8) is no longer needed and is removed from memory.

After operations 84 and 85 have been completed, the computer performs operation 86 wherein a new value of slice height set point (h is calculated in response to the previous value therefor (h and the calculated value of headbox slice set point change Ah The value of headbox slice set point is read out after operation 86 has been completed to control the actuation, if necessary, of slice actuator 61 to change the height of slice opening 13.

To provide a better and more complete understanding of the operation of the present invention, reference will now be made to FIGS. 3 and 4. In FIGS. 30 -30, there are disclosed wave-forms indicative of the response of a headbox to step changes in the set point of a stream valve and the resulting variation in the velocity of a jet spouting from the slice opening of a headbox that does not include the coordinated control apparatus and method of the present invention. The step change in stream valve set point, indicated in FIG. 3a at time T occurs, for example, in response to an operator induced change to vary dry line position.

In response to the step change in stream valve setting occurring at time T the total head in the headbox has a tendency to increase as an exponential like function, as

indicated in FIG. 3b. In response to the tendency of the headbox head to increase, the velocity of the jet spouting from the slice opening also has a tendency to increase as an exponential like function, as indicated in FIG. 3c. The increase in the spouting velocity of the jet emerging from the slice opening, however, results in a rush-drag change, whereby the formation properties of the sheet formed on the wire have a tendency to change.

To prevent the velocity of the jet spouting from the slice opening and the total head within the headbox from increasing in response to the step change in stream valve setting as indicated in FIGS. 3a, 3b and 3c, the height of the slice opening should be increased in a step manner as illustrated in FIG. 3d. By increasing the height of slice opening 13 as indicated in FIG. 3d, the total amount of liquid in headbox 11 remains constant and the total head remains constant, as shown in FIG. 32. By maintaining the headbox total head constant, the velocity of the jet spouting from the slice opening does not change, whereby the rush-drag between the spouting jet and wire does not vary, to preclude differences in the formation properties of the sheet being formed.

The ideal situations illustrated by FIGS. 30, 3d and 3e are difficult to attain because of the lag in the movement of the headbox pond to changes in stream valve and slice opening settings, the different response times of the slice opening and the stream valve and the necessity for filtering the total head signal derived by pressure transducer 24. Also, it is usually not considered necessary to provide perfect compensation of the type illustrated by FIGS. 3d and 3e to prevent sufficiently accurate control of the formation of the paper sheet.

The system of the present invention does not attempt to provide exact compensation, as illustrated by FIGS. 3d and 3e, but provides an approximation thereof. In FIGS. 4a-4d, the operation of the system of the present invention is considered, assuming that a step change in stream valve 22 occurs at time T three seconds before the beginning of a fifteen cycle sampling time at T for setting the value of H to H". The step change in stream valve 22 is assumed to cause a positive increase in stream flow and results in a negative step change at time T as indicated in FIG. 4a, in the rush-drag set point, R' calculated during operation 78. In response to the step change in the rush-drag set point at time T a rush-drag error is calculated during operation 76 and is refluxed during operation 77 as a positive step change for the set point for slice opening 13. All of these operations occur during the cycle time ending at T During the next computer cycle time ending at T the rush drag set point is returned to the desired value therefor due to coupling of the AR calculation of operation 76 to operation 78 during the cycle time ending T The value of LR' remains constant after T as shown in FIG. 4a until the next sampling time T occurs because AR, remains constant over this interval.

In response to the step change in slice opening set point calculated during operation 77, the height of the slice opening is increased as indicated in FIG. 4b. The slice opening is still translating toward a new set point value therefor when sampling interval T occurs.

In response to opposite effects of increased stream flow occurring at T and the change in the height of slice opening 13, the head in headbox 11 has a tendency to vary, in an ideal case, in the manner indicated by FIG. 40. The initial, positive going portion of the curve of FIG. 4c occurs in response to the increasing fluid level of the pond of headbox 11 resulting from the increased stream fiow through valve 22. The negative going portion of the curve of FIG. 40 results from the increased flow from the slice opening, as shown in FIG. 4b and does not occur simultaneously with the positive going changes because of the response of the slice opening being slower than that of the stream valve. The positive and negative going portions of the curve of FIG. 40 are equal to each other,

whereby under ideal conditions the total head in the headbox returns, towards the end of the sampling interval bounded by T and T to the same initial value it had prior to the step change in stream flow occurring at time T The ideal curve of FIG. 40, however, does not actually occur because the value of total headbox head, H, sampled at time T is displaced from the final desired value thereof. In addition, the opening of slice 13 increases at a slow rate and does not reach the new set point for it until several seconds after the sampling time T in the example illustrated in FIG. 4b the slice height reaches the set point value about five seconds after T Because the sampled value of H at T is not zero, the value of AR calculated in operation 76 has a finite, nonzero value. During the cycle time, T immediately after the sampling time, the value of R calculated during operation 78 is reduced by the finite value of AR calculated at T as indicated in FIG. 4a. R' is modified by the value of AR at T and is returned to R at T because operation 75 is entered at that time. In response to the value of H existing at T there is a tendency to over correct for the disturbance to stream valve 22 made at T This is because the slice has not been moved sufficiently to compensate for the effect of the increased stream flow that caused the increased headbox head.

T o avoid the over correction problem, the present invention subtracts out in operations 84-86 the slice set point changes made during the last six cycle times preceding T In the example, the finite value of Ah calculated during cycle time T is subtracted from Ah during operation 83, which occurs only during the sampling interval T The modified value of hsLSET calculated during operation 83 changes the value of hsLSET computed in operation 86 during cycle times T and is not varied in response to the disturbance occurring at T5 until T515- The effect of this series of operations is shown in FIG. 4d wherein the actual filtered head, H", in headbox 11 is illustrated. At time T the filtered head has the value H',. The head increases slightly thereafter and then decreases below H' to a value of H which is above H' the initial and final desired value thereof. The value of H' is reached because of the algebraic effects of H' at T51 and Ah at T1- In response to the step change at T the set point for slice height increases at T to the value h illustrated in FIG. 4b. At T however, the slice height set point decreases to hsLSET2 because of the effect of operation 83 and the effect of H on operation 76. A few seconds after T the slice height stabilizes to h as indicated in FIG. 4b. It is to be noted that the desired head and slice opening are not reached during the interval T to T but a considerable compensation has been made for the tendency of the system to overcorrect. A few seconds after T the desired head and slice opening values are reached as shown by FIGS. 4a and 4d and the desired rush drag relationship has been restored.

It is to be understod that the example given is to enable an understanding of the operation to be obtained. In actuality, the system is generally employed to prevent over correction in response to operator or grade change variations in wire speed or rush drag set point, as well as disturbances to the head from other sources within the machine, e.g., air leaks in the headbox.

Because of the relatively straightforward manner by which the system operates in response to disturbances occurring during the first half of a sampling interval, no detailed description thereof is provided.

While there has been described and illustrated one specific embodiment of the invention, it will be clear that variations in the details of the embodiment specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims. For example, in certain systems the speed of wire 14 can be considered constant and the value thereof set into the computer memory on an a priori basis. It may also be possible to substitute computer 67 and the calculations made thereby with an operator who reads meters associated with the various transducers and manually adjusts some of the different actuators.

I claim:

1. A system for controlling the operation of a headbox of a machine producing a fibrous sheet from a slurry of fiber and water fed to a moving drainage means, said headbox including a slice opening for feeding a jet of the slurry to the drainage means and means for controlling the size of said slice opening, said system comprising means for deriving a first signal indicative of the flow rate of slurry to the headbox, means for deriving a second signal indicative of the total head of fluid in the headbox, means receiving said first and second signals for deriving a slice opening control signal responsive thereto and means for applying the control signal to said means for controlling.

2. The system of claim 1 further including means for deriving a third signal indicative of the speed of the drainage means, said control signal deriving means including means responsive to the first, second and third signals for deriving a fourth signal indicative of the change in slice opening necessary to achieve a predetermined relationship betweenvelocities of a jet emerging from the slice and the drainagemeans, said control signal applying means being responsive to said fourth signal.

3. The system of claim 2 further including means for enabling the control signal to be varied only on response to at least a predetermined value of the fourth signal.

4. The system of claim 2 further including means responsive to the second signal for controlling the level of liquid in the headbox.

5. The system of claim 2 further including means having a manual control for changing the flow rate of the slurry fed to the headbox without changing the flow rate of fiber fed to the headbox.

6. The system of claim 2 further including means having a manual control for changing the flow rate of the slurry fed to the headbox without changing the flow rate of fiber fed to the headbox, and means responsive to the second signal for controlling the level of liquid in the headbox.

7. The system of claim 1 wherein said control signal deriving means includes means: for providing a dynamic model of the response of the headbox to changes in total headbox head and the flow rate of the slurry fed to the headbox to derive an indication of the amount of movement of the slice opening required to maintain a predetermined relationship between the velocities of the jet and drainage means, and for comparing the indication derived from the dynamic model with an indication of a predetermined fraction of the existing slice opening.

8. The system of claim 2 wherein the control signal deriving means includes means for periodically deriving the fourth signal indication of the required slice opening movement at recurrent first time intervals and for periodically deriving a filtered total head indication from the second signal at recurrent second time intervals greater than the first time intervals; for accumulating said indicated changes in the slice opening only over a predetermined number of the first time intervals but for a time less than one of the second time intervals; and for subtracting the accumulated changes from the most recently derived indicated change in the slice opening, thereby to compensate said control signal for changes in the total head occurring as a result of slice movement during said predetermined number of the first time intervals.

9. A method of controlling the operation of a headbox of a machine producing a fibrous sheet from a slurry of fiber and water fed to a moving drainage means, said headbox including a slice opening for feeding a jet of the slurry to the drainage means, comprising the steps of deriving respective signals indicative of the velocity of the drainage means, the total head of fluid in the headbox, and the flow rate of the slurry fed into the headbox, producing a control signal responsive to all of said signals, and adjusting the slice opening in response to said control signal so as to maintain a predetermined relationship between the velocities of the jet and the drainage means.

10. The method of claim 9 further including the step of changing the position of a dry line on the drainage means by altering the flow rate of slurry fed to the headbox without changing the flow rate of fiber fed to the headbox.

11. The method of claim 10 further including the step of controlling the level of liquid in the headbox.

12. A system for controlling the operation of a headbox of a machine producing a fibrous sheet from a slurry of fiber and water fed to a moving drainage means, said headbox including a slice opening for feeding a jet of the slurry to the drainage means and means for controlling the size of said slice opening, a plurality of actuators for adjusting, respectively, the flow rate of slurry fed to the headbox, the extent of the slice opening and the velocity of the drainage means, said system comprising first, second and third means for deriving first, second and third signals respectively indicative of: the flow rate of the slurry fed to said headbox, total head in the headbox, and the velocity of the drainage means; means combining said signals for deriving a control signal to drive the one of said actuators afiecting the extent of the slice opening thereby to aflect the relative velocities of the jet and drainage means, and means for applying only changes in the control signal above a predetermined amplitude to the actuator.

References Cited UNITED STATES PATENTS 3,293,120 12/1966 Harman, Jr. et al. 162-259 X 3,490,689 1/1970 Hart et al. 162--252 X OTHER REFERENCES McKnight, M. A.: Paper Technology, vol. 7, #1 (1966), pp. 45-52.

Savas, E. S.: TAPPI, vol. 47, #5 (May 1964), pp. 127A-133A.

Zahradnik et al: Instruments and Control Systems, vol. 38 (August 1965), pp. -58.

Horrocks, T.: Proc. IEE, vol. HI, #11 (November 1964), pp. 1894-1906.

S. LEON BASHORE, Primary Examiner A. DANDREA, IR., Assistant Examiner U.S. Cl. X.R. 

